The present invention relates to monitoring and testing of multiplexed communication networks and, more particularly, to a method and system for selectively accessing desired information in a multiplexed data stream without interrupting the transmission of the data stream to thereby provide for non-disruptive monitoring and testing of transmission paths and equipment serving subscribers in a time division multiplexed data transmission system.
Various systems have been proposed and are presently in use for transmitting data produced by a large number of subscribers from one location to another distant location. These systems almost invariably employ time division multiplexing to combine the data into one multiplexed data stream. The multiplexed data stream is then typically transmitted over transmission networks which include telephone lines, telephone switching equipment and radio transmission (microwave) links.
One system presently in use is referred to as the Dataphone Digital Service (DDS). In such a system, a central office typically serves a large number of local data subscribers. Equipment at the subscribers' stations transmits and receives data over lines connecting the subscribers' stations to the central office. At the central office, the data received from the subscribers' stations is multiplexed and the multiplexed data stream is then transmitted from the central office to a distant central office or, in some situations, to a nearby location, where the data is eventually demultiplexed and transmitted to its ultimate destination.
In one typical common carrier system, the multiplexed data stream transmitted from the subscriber's central office is transmitted at a bit rate of 1.544 megabits per second and the data stream may contain data from up to 460 subscribers. The multiplexing is typically accomplished in two stages with a subrate data multiplexer (SRDM) accepting and multiplexing data from up to 20 subscribers and with a T1 rate data multiplexer (T1DM) accepting and multiplexing up to 23 SRDM channels.
As with data transmitted from the central office serving the local data subscribers, the data received by the central office from other central offices is received in a multiplexed format and is demultiplexed for local transmission to the local subscribers. In this manner, the numbers of very long transmission links between two central offices are minimized since as many as 460 subscribers may be served in a single multiplex of which many can be carried on a transmission link.
Present procedures for testing the data links or communication paths serving each individual subsciber are time consuming and require considerably equipment at each central office. The prevalent manner of testing data links requires access to the network at points preceding the multiplexing of the subscriber data and therefore requires test jacks for each individual subscriber and a pair of test units which can be moved from jack to jack to test the subscriber data links individually. Thus, in a central office serving thousands of subscribers, a test panel having thousands of jacks must be provided. The space consumed by the test jack panel alone can therefore be considerably in one of the larger central offices of a typical common carrier system. Moreover, since this prevalent testing technique requires an operator in insert the test unit connectors in the test jacks on an individual basis and operate the test sets, the testing of all of the subscriber lines may be an enormously time consuming task.